The invention relates in general to a catalyst and process for the isomerization of alkyl aromatics. In particular, the invention relates to improvements in the commercial processes in which a mixture of xylenes and ethylbenzene are reacted along with hydrogen over a platinum-containing catalyst to produce a near equilibrium mixture of xylenes while converting ethylbenzene to xylenes.
Isomerization of alkyl aromatics has become particularly important commercially. In processes for the production of xylenes, the ortho and paraxylenes are the preferred products. Paraxylene is principally used in preparation of polyesters while orthoxylene's main end use is in preparation of phthalic anhydride. Metaxylene has fewer important end uses and thus it may be converted to the para and ortho forms, which have greater commercial value. After paraxylene and orthoxylene have been separated as products, it is typical that metaxylene is recycled to the isomerization reactor along with unreacted ethylbenzene, residual amounts of orthoxylene and paraxylene, and naphthenes. Thus the combined isomerization and separation steps produce the desired products and the remaining C.sub.8 compounds are recycled to extinction.
Ethylbenzene is difficult to separate from the xylenes since their boiling points are very close. Accordingly, ethylbenzene is normally present within the mixture of xylenes prepared by extraction or distillation from a hydrocarbon stream. There are two approaches commonly taken in the prior art to handle the ethylbenzene contained in a mixed xylene stream, which may be present in the range of 15-65 wt.%, which is greater than the equilibrium amount. Generally, ethylbenzene is not easily isomerized and many processes have chosen to remove the ethylbenzene by destroying it through disproportionation, hydrodealkylation or the like to yield lighter and heavier compounds which can be easily separated by distillation from the C.sub.8 compounds. It will be apparent that such processes dispose of a significant quantity of potential xylenes by such reactions. Typical processes of the prior art which remove ethylbenzenes by destruction are U.S. Pat. Nos. 3,856,871-4 in which special zeolite molecular sieves are used to isomerize xylenes while converting ethylbenzene to C.sub.6, C.sub.7, C.sub.9, C.sub.10 aromatic compounds and non-aromatics.
It is also known in the prior art to react ethylbenzene to form xylenes in the presence of hydrogen and a hydrogenation-dehydrogenation catalyst, preferably platinum on alumina. Typical of the prior art is U.S. Pat. No. 2,976,332 which discloses a catalyst comprising platinum on alumina plus an amorphous silica alumina to convert ethylbenzene and isomerize xylenes, with a minimum of side reactions which reduce yield selectivity and contribute to catalyst aging. Platinum provides the hydrogenation-dehydrogenation function believed to be required for the probable reaction mechanism for converting ethylbenzene. Catalysts of this type have been commercially used with considerable success.
When ethylbenzene is not present, conversion of xylenes alone has been found to be possible with many catalysts, which include an amorphous silica alumina, hydrogen mordenite, dealkalized mordenite, and special zeolites. In the case of the zeolites, it has been common in the prior art to prepare zeolites in the sodium form and thereafter to replace sodium with other cations or hydrogen ions in order to improve performance.
Combination catalysts which are capable of isomerizing xylenes and ethylbenzenes to approach an equilibrium distribution of isomers include that of U.S. Pat. No. 2,976,332 mentioned above which combines platinum on alumina with amorphous silica alumina. More recent combination catalysts are disclosed in U.S. Pat. No. 3,409,686 in which alumina gel is mixed with particles of hydrogen mordenite to form a mixed base which is, after drying, impregnated with a platinum solution. Another mixed catalyst is illustrated in U.S. Pat. No. 3,767,721 in which platinum on alumina in a fine powder form is combined with powdered mordenite. It is shown that hydrogen mordenite in such a combination is overly active and promotes destructive reactions which are undesirable. Accordingly, the patent discloses and claims a process in which the catalyst is a partially dealkalized mordenite, rather than hydrogen mordenite. The content of alkali and alkaline earth metals in the mordenite is adjusted so that destructive reactions are minimized.
In summary, the prior art discloses a number of catalysts for isomerization of xylenes alone and combination catalysts for isomerization of xylenes combined with conversion of ethylbenzenes to xylenes. Specifically with relation to the present invention, the prior art teaches that hydrogen mordenite is useful for isomerization of xylenes. In addition, the art teaches that hydrogen mordenite may be combined with an alumina gel to form a mixed base, which is thereafter inpregnated with a platinum solution. Another catalyst in the prior art employs a segregated base wherein the platinum is deposited only on the alumina portion and thereafter the platinum on alumina is combined with a dealkalized form of mordenite rather than hydrogen mordenite. In such a catalyst, the art teaches that the hydrogen form of mordenite produces an overly active catalyst. Finally, the prior art teaches that a segregated base may also be used in which platinum is deposited on an alumina and thereafter combined with amorphous silica alumina, both being in the finely divided powder form.
While some of the prior art catalysts have been successful in isomerizing xylenes and ethylbenzene, further improvement has been desired in order to achieve a highly selective catalyst which can be operated to approach chemical equilibrium in the isomerization of xylenes and ethylbenzene while at the same time avoiding the destructive reactions which can result in a loss of C.sub.8 aromatics. In addition, maintenance of activity and selectivity for longer periods than heretofore possible is desirable. Such an improved catalyst is described below.